These hydrates can form when the water is in presence of light hydrocarbons, either in the gas phase or dissolved in a liquid phase, such as a liquid hydrocarbon, and when the temperature reached by the mixture becomes lower than the thermodynamic hydrate formation temperature, this temperature being given for a gas composition for a given pressure value.
In order to decrease the production cost of crude oil and of gas, on the investment level as well as on the operating cost level, it can be considered, for example in the case of offshore production, to reduce or even eliminate any processings applied to the crude oil or to the gas that is to be transported from the reservoir to the coast, and notably leaving all or part of the water in the fluid to be transported. The effluents are thus transferred in the form of a multiphase flow by means of a pipe to a processing platform. This procedure is particularly advantageous when offshore production is performed in a difficultly accessible area. However, it has a notable drawback due to the risk of formation of hydrates.
In fact, oil effluents containing a gas phase and a liquid phase can be made up for example of a condensate gas, a natural gas or an associated gas mixed with crude oil. They are generally saturated with water and can even contain free water in some cases.
When these effluents are transported at the sea bottom, it may happen that, as a result of the lowering of the temperature of the effluent produced, the thermodynamic conditions are likely to promote the formation of hydrates which agglomerate and block the transfer lines. The sea bottom temperature can be low, for example of the order of 3.degree. or 4.degree. C.
The cooling of such a mixture can lead to the formation of hydrates which are inclusion compounds, the water molecules congregating to form cages in which light hydrocarbon molecules, such as methane, ethane, propane, isobutane are trapped. Some acid gases present in natural gas, such as carbon dioxide or hydrogen sulfide, can also form hydrates in the presence of water.
Conditions promoting the formation of hydrates can also be encountered in the same way on land for lines buried only a short distance from the ground surface, when for example the temperature of the ambient air is rather low, notably in northern areas such as the arctic zones.
The agglomeration of hydrates causes the filling and the blocking of the transfer lines as a result of the formation of plugs which prevent the passage of crude oil or of gas and can lead to a production stop that is often quite long because the decomposition of the hydrates formed is very difficult to achieve and therefore brings about considerable financial losses.
In order to avoid such drawbacks, various methods are described in the prior art.
A processing intended to remove the water can be carried out on a surface platform situated in proximity to the reservoir, so that the effluent, initially warm, can be processed before the hydrate formation conditions are met on account of its cooling with the sea water. This solution however requires to bring the effluent back up to the surface prior to transferring it towards a main processing platform and to have an intermediate processing platform.
The transfer line for transporting the effluent can be insulated or even heated by means of an appropriate device, such as the device described in patent application WO-90/05,260, in order to prevent too fast a cooling of the fluids transported. However, such devices are costly and complex in their technical realization.
Patent HU-186,511 teaches that an electromagnetic wave having selected frequency values and propagation modes can be sent out to make the formed hydrates melt.
Patent SU-442,287 teaches to use an ultrasonic wave to break the hydrate crystals and to free the trapped gas thereby.
It is also well known to add permanently additives allowing to inhibit the formation of hydrates or to reduce them in the dispersed form. Such a technique is very costly.
The methods described in the prior art have notable drawbacks notably on account of the high energies necessary to act upon the crystals already formed and/or of the use of costly products which often have to be separated.